Method of using household waste in the production of concrete

ABSTRACT

There is shown a method of treating household waste comprising separation, shredding and dewatering steps, as well as detoxifying steps, prior to mixing the household waste with an alkali solution and thereafter mixing the alkali solution and household waste mixture with a dry mixture of cement and aggregate to form a concrete mixture. The concrete mixture can then be used in the construction of building elements or may in turn be used as an aggregate material in future concrete mixtures. Expensive composting steps are thereby avoided and the household waste is handled in an environmentally secure, yet efficient manner.

This invention relates to a method of using household waste in theproduction of concrete. Reference is made to the applicants ownco-pending Irish Patent Application No. 2002/0831 and European PatentApplication Number 02394102.4 both entitled “A method of usingwastewater sludge in the production of concrete”, having the samepriority date as the present application, the content and disclosurethereof being incorporated herein by reference.

Nowadays, people are becoming more aware of the need to recycle theirrefuse in an environmentally friendly manner. Local authorities andother companies that have been entrusted with refuse disposal have begunimplementing extensive recycling programmes whereby much of the refuseto be disposed of is separated off and sent to dedicated recyclingplants for re-use. Typically, glass, paper and other such products havebeen separated from the remaining refuse and sent for recycling. Otherwaste such as household waste and in particular biodegradable householdwaste is usually subjected to further treatments before being sent tolandfill, incineration or dumping at sea. Generally speaking though,these methods of disposal are often relatively expensive and variousseparate environmental issues arise from each method of disposal.

Another solution suggested for the treatment of biodegradable householdwaste is composting. The waste is stored for a prolonged period of time,usually over six weeks, and allowed to decompose. The decomposed wastemay then be used as compost for agricultural or horticultural purposes.This allows for the waste to be recycled in an environmentally friendlymanner. There are, however, problems associated with this known methodof recycling biodegradable household waste. The first problem with thismethod is the noxious smell that is released by the biodegradablehousehold waste as it decomposes. This can be quite overpowering andoften leads to complaints and disputes with neighbouring residents. Tocontain the noxious smell from the decomposing matter, the composting isfrequently carried out in negative pressure sheds so that the smellaffects the surrounding environment as little as possible. However,these sheds are quite expensive to provide and further increase the costof recycling the biodegradable household waste. Furthermore the entireprocess is quite lengthy and time consuming to complete.

It is an object therefore of the present invention to provide a methodof recycling household waste and in particular, biodegradable householdwaste in an environmentally friendly manner that is both simple andinexpensive to implement and that overcomes at least some of thedifficulties of the prior art.

STATEMENTS OF INVENTION

According to the invention, there is provided a method of treatinghousehold waste comprising the steps of:

-   -   (a) separating any non-biodegradable waste above a predetermined        size from the household waste for subsequent disposal;    -   (b) shredding the remaining household waste in a shredder;    -   (c) mixing the shredded household waste with an alkali solution        to form an alkali and household waste mixture having a pH above        11.5;    -   (d) separating any remaining metallic waste from the alkali and        household waste mixture for subsequent disposal;    -   (e) separating any wood or plastics material from the alkali and        household waste mixture for subsequent disposal;    -   (f) removing any gases emitted by the alkali and household waste        mixture;    -   (g) subjecting the alkali and household waste mixture to a        centrifuge to form sludge cake having a liquid content of        between 60% and 95%, and untreated water; and    -   (h) mixing the sludge cake with a dry mix of aggregate and        cement to form a concrete mixture.

This is seen as a particularly efficient method of treating householdwaste as the non-biodegradable waste is removed gradually from thehousehold waste, leaving the biodegradable waste that was heretoforedifficult to dispose of in a safe and cost efficient manner. Extensivecomposting of the biodegradable waste will not have to be carried outwhich will therefore negate the need for storing the compost for asignificant period of time, as well as having to provide negativepressure sheds and the like to contain the smell emanating from thedecomposing biodegradable waste. The waste is neutralised and containedwithin a concrete mixture in a quick and efficient manner. By creatingan alkali and household waste mixture having a pH of greater than orequal to 11.5, the vast majority of any harmful bacteria contained inthe biodegradable waste will be eliminated from the household waste,thereby helping to provide a concrete mixture that is bothenvironmentally secure and may be used in public construction projects.Furthermore, any viruses present in the concrete will be killed off andthe risk of subsequent leeching from the concrete of harmful materialsis much reduced by having the pH of the alkali and household wastemixture above 11.5. By passing the household waste through the abovesteps sequentially, the household waste may be separated out in a quickand efficient manner, requiring the minimum of human intervention,thereby providing a cost efficient method of treating the householdwaste.

In another embodiment of the invention, the step of separating anyremaining metallic objects from the alkali and household waste mixturefor subsequent disposal further comprises passing the alkali andhousehold waste through an eddy magnet. This is seen as a particularlyefficient and cost effective way of removing the remaining metallicobjects from the household waste. These metallic objects may then bestored separately for onward recycling.

In one embodiment of the invention, the step of separating any wood orplastics material from the alkali and household waste mixture forsubsequent disposal comprises passing the alkali and household wastemixture through a floatation tank and skimming the wood and plasticsmaterial from the floatation tank. This is seen as particularlyefficient as the wood and plastics materials may be removed from theother biodegradable household waste which is the major cause of theundesirable smells, as well as being the source of most diseases, andthe wood and plastics which are reasonably innocuous may be passedonwards for recycling.

In a further embodiment of the invention, the further step is carriedout of passing the untreated water to a water treatment plant forsanitation and recycling. This water may be used for more usefulpurposes such as general water supply or in the production of concrete,further down the line.

In another embodiment of the invention, the additional step is carriedout of pouring the concrete mixture into plastic containers to preventinadvertent leaching of contaminants from the concrete mixture. This isseen as a particularly efficient way of preventing any leeching ofcontaminants from the concrete mixture that may still remain after thehousehold waste has been turned into concrete.

In one embodiment of the invention, the gases removed from the alkaliand household waste mixture are passed to a burner for burning. This isa useful way of handling the gases in a clean and efficient manner thatis cost effective in its implementation and may even contribute toheating or powering of the place in which the method is being carriedout

In one embodiment of the invention, the step of mixing the shreddedhousehold waste with an alkali solution further comprises mixing theshredded household waste with an alkali solution having a pH equal to orabove 12.5. Alternatively, the alkali solution could have a pH above 13.Furthermore, the alkali solution could have a pH above 13.5. By having ahigh pH alkaline solution, a more efficient kill of bacteria in thebiodegradable matter is achieved and a concrete that will not presentenvironmental danger will be provided.

In a further embodiment of the invention, the sludge cake is added insufficient quantities so that the sludge cake forms between 5% and 50%by weight of the concrete mixture. This is a useful and efficient way todispose of the sludge cake, while still maintaining the strength anddurability characteristics of concrete that is acceptable in the use ofbuilding materials. Alternatively, the sludge cake may be added insufficient quantities so that the sludge cake forms between 10% and 40%by weight of the concrete mixture. Alternatively, the sludge cake couldbe added in sufficient quantities so that it forms between 15% and 30%by weight of the concrete mixture. It is important that a concrete ofsufficient durability and strength is produced.

In another embodiment of the invention, the aggregate and cement aremixed together in a separate container prior to mixing with the sludgecake. By mixing the aggregate and cement in a separate container priorto mixing with the sludge cake, the alkali may work on the harmfulbacteria contained in the sludge cake, without having to act on othersubstances at the same time. This will enhance the usefulness andeffectiveness of the alkali solution in killing bacteria in the sludgecake.

In one embodiment of the invention, the step of mixing the shreddedhousehold waste with an alkali solution further comprises mixing theshredded household waste with a concrete hardener. It has been foundthat a concrete hardener may act as an alkaline solution and killbacteria present in the biodegradable household waste. Furthermore, theconcrete hardener will help to harden the concrete mixture in due courseand will not have a detrimental effect on the quality of the concreteproduced.

In a further embodiment of the invention, the additional step is carriedout of adding a bonding agent to the concrete mix. It is envisaged thatthe bonding agent may have a pH in the region of 8 to 11. The bondingagent used may be carboxylated styrene butadiene alkali. By adding abonding agent to the concrete mix, there will be provided betteradhesion of the component particles in the concrete, also improving thepH properties of the concrete.

In another embodiment of the invention, the aggregate comprises one ormore of grey wacke stone, sand, sandstone, gravel, limestone, crushedshale, crushed seashells, pencil, kiln dried sand, grit, pulverised fuelash, slag from steelworks, and recycled crushed concrete.

In one embodiment of the invention, the method further comprises theadditional step of allowing the concrete mixture to set and after apredetermined length of time, crushing the concrete mixture forsubsequent re-use as an aggregate in the method. This is seen as auseful way of recycling more biodegradable household waste. It is alsovery efficient in reducing the costs of the production of the concretemix, as previously recycled biodegradable household waste will have beenused in its production.

In one embodiment of the invention, the method further comprises thestep of adding water to the concrete mixture, on addition of the sludgecake to the cement and aggregate mixture. This will ensure that a goodmixture of concrete will be formed that is both strong and durable.

In a further embodiment of the invention, the method further comprisesthe step of adding a detergent to the concrete mixture prior to curing.By adding a further detergent to the concrete mixture, the bacteria killwill be enhanced, thereby providing a concrete that is moreenvironmentally secure and will not leech contaminants into the soilafter curing.

In another embodiment of the invention, the sludge cake, cement andaggregate are mixed in a ratio of 1:1:6 by weight to form the concretemixture. This is seen as a useful ratio of sludge cake, cement andaggregate to use in the concrete mixture, that will provide a concretethat is both strong, durable and can be used in varied applications.

In a further embodiment of the invention, the method further comprisesthe step of milling the household waste prior to separating any wood orplastics material therefrom. This will further enhance the separation ofthe wood and plastics material from the remaining biodegradablehousehold waste, as the household waste will be ground into a finegranular material that will separate quicker and more evenly in afloatation tank.

In one embodiment of the invention, the step of mixing the sludge cakewith the cement and aggregate to form a concrete mixture is performed inthe mixing drum of a concrete mixing truck. This is seen as aparticularly useful way of mixing the sludge cake with the cement andaggregate, as additional drums would not have to be provided and theaggregate and cement can be premixed before the introduction of thesludge cake. The concrete mixture may then be brought to a desireddestination before being poured and setting on site.

In a further embodiment of the invention, the sludge cake has a solidscontent of between 10 and 40%. Ideally, the solids content will bebetween 10 and 25%. In this way, the household waste will be able toprovide sufficient water to produce concrete and additional water willnot be required in its production.

In one embodiment of the invention there is provided a method of usinghousehold waste in the production of concrete comprising the steps ofmixing cement, aggregate and household waste together to form a concretemix, characterized in that the method further comprises the initialsteps of:

-   -   (a) shredding the household waste to a predetermined particle        size;    -   (b) mixing an alkali solution with the shredded household waste        to form an alkali and household waste mixture having a pH equal        to or greater than 11.5, prior to mixing with the cement and the        aggregate.

This is seen as a particularly useful method of using household waste inthe production of concrete that is both efficient and cost effective. Byusing this method there will be no need for expensive incineration stepsto be carried out as once an alkali and household waste mixture having apH equal to or greater than 11.5 is produced the majority of the harmfulbacteria contained in the household waste will be eliminated by thestrong reaction. The remainder of the harmful bacteria. If any, will beneutralized by trapping the bacteria in the concrete product. Theconcrete product produced will be suitable for most buildingapplications and will satisfy even the most stringent health and safetylegislation. The method provides a very simple and inexpensivealternative to methods in which incineration, extensive pre-treatment orcomposting steps are required that slow down the process as well asadding to the overall cost of the process.

In another embodiment of the invention there is provided a method ofusing household waste in the production of concrete in which the alkalisolution is added to the shredded household waste so that the alkali andhousehold waste mixture has a liquid content by weight of between 60%and 90%. This is seen as beneficial as by having an alkali and householdwaste mixture with such a liquid content, further water will not have tobe added to the cement and aggregate when the concrete is beingproduced. Furthermore, the alkali solution will be very effective insuch an aqueous solution in breaking down the bacteria present in thehousehold waste quickly and effectively.

In a further embodiment of the invention there is provided a method ofusing household waste in the production of concrete in which the alkalisolution is added to the shredded household waste so that the alkali andhousehold waste mixture has a liquid content by weight of between 75%and 90%. This is seen as particularly beneficial as a very highpercentage of all bacteria contained in the household waste will beeliminated almost instantly and there will be sufficient liquid contentfor the subsequent production of concrete.

In one embodiment of the invention there is provided a method of usinghousehold waste in the production of concrete in which the methodfurther comprises the steps of:

-   -   (c) passing the alkali and household waste mixture through a        centrifuge to separate the alkali and household waste mixture        into sludge cake having a liquid content by weight of between        65% and 90%, and untreated wastewater;    -   (d) drawing off the untreated wastewater and leaving only the        sludge cake; and    -   (e) mixing the sludge cake with the cement and aggregate        mixture.

This is seen as a particularly useful way of eliminating a significantproportion of the wastewater from the alkali and household waste mixturethat will not be required in the subsequent concrete mixing step. Thewastewater is at this stage a reasonably safe and may be disposed of byalternative measures.

In another embodiment of the invention there is provided a method ofusing household waste in the production of concrete in which the alkaliand household waste mixture are passed through a centrifuge until thesludge cake has a liquid content of between 75% and 85%. This is seen asa sufficient amount of liquid to aid in the subsequent concrete makingstep yet not requiring the addition of any further water in the concretemixing step. At the same time, any unnecessary wastewater is drawn offthe sludge cake.

In a further embodiment of the invention there is provided a method ofusing household waste in the production of concrete in which thehousehold waste is shredded to provide a shredded household waste havingan average particle size of between 1 mm and 10 mm. Preferably thehousehold waste is shredded to provide a shredded household waste havingan average particle size of between 3 mm and 8 mm. This is seen as auseful particle size as it is easier for the alkali solution to breakdown and will more readily mix with the alkali solution. Furthermore,this particle size will also form a robust concrete mixture that willform durable concrete that may be used in a variety of applications.

In one embodiment of the invention there is provided a method of usinghousehold waste in the production of concrete in which the alkalisolution added to the household waste is a concrete hardener. This willfurther enhance the effectiveness and usefulness of the concreteproduced by the method as the concrete will harden faster and lengthysetting times will not be required while at the same time theeffectiveness of the bacterial kill will not be diminished.

In another embodiment of the invention there is provided a method ofusing household waste in the production of concrete in which the methodfurther comprises the step of adding a bonding agent to the concretemix.

In a further embodiment of the invention there is provided method ofusing household waste in the production of concrete in which the step ofadding an alkali solution to the shredded household waste furthercomprises the step of adding an alkali solution having a pH equal to orabove 12.5. This will ensure that a high initial bacterial kill isachieved when the alkali solution is added to the household waste. Thestrong alkali will be effective very shortly after it has beenintroduced.

In one embodiment of the invention there is provided a method of usinghousehold waste in the production of concrete in which the alkali andhousehold waste mixture comprises between 5 and 50% by weight of theconcrete mixture. Alternatively, the alkali and household waste mixturecomprises between 10% and 40% by weight of the concrete mixture.Preferably, the alkali and household waste mixture comprises between 15%and 30% by weight of the concrete mixture. This will provide a strongand durable concrete that may be used for numerous applications while atthe same time providing a concrete that will contain a high amount ofhousehold waste therein that is disposed of simply and efficiently.

In another embodiment of the invention there is provided a method ofusing household waste in the production of concrete in which the alkaliand household waste mixture, cement and aggregate are mixed in the ratio1:1:6 by weight to form the concrete mixture. This will further providea strong concrete that aids the disposal of a high proportion ofhousehold waste.

DETAILED DESCRIPTION OF THE INVENTION

The invention will now be more clearly understood from the followingdescription of some embodiments thereof, given by way of example only,with reference to the accompanying drawings, in which:—

FIG. 1 is a diagrammatic view of a typical plant layout suitable forperforming the process according to the invention,

FIG. 2 is a diagrammatic illustration of one form of suitable apparatusthat may be used for carrying out the invention; and

FIG. 3 is a block diagram of the process of recycling household wasteaccording to the invention.

Referring to the drawings, and initially to FIG. 1 thereof, there isshown a diagram of a typical plant layout suitable for performing themethod according to the invention. Refuse is taken in, in rubbish intakearea 40. The refuse is stored in storage area 41 until ready forprocessing. When desired, the refuse is passed to hopper/bag ripper 42and thereafter placed on conveyor belt 43. An over belt magnet 46extracts any large pieces of metal and deposits them to metal skip 47.Further undesirable pieces of refuse are manually separated at pickingstation 49. The refuse then proceeds along the conveyor belt to shredder50 where the refuse is shredded into smaller particles. The shreddedrefuse continues along conveyor to an over belt Eddy magnet 52 whichcollects further metal objects such as aluminium cans and deposits theminto skip 53.

The remaining refuse passes through finger screen blower and sucker 55and onwards to a high speed mill/shredder 56. Once this is complete theshredded material is passed to the floatation tank 58 where apredetermined quantity of alkali solution is added to the shreddedhousehold waste to form an alkali and household waste mixture having apH above 11.5. Any plastics and wood present in the alkali and householdwaste mixture are then skimmed from the mixture in the floatation tank58. The plastics and wood collected are passed to a dedicated skip 59.The remaining shredded refuse continues to sealed gas extractor tank,with skimmer and agitator, 61. Gases from the sealed gas extractor tank61 are fed off to gas fire burner generator 62 for burning and thetreated alkali and household waste mixture is fed through pipe 63 tocentrifuge 65 to form sludge cake having a liquid content of between 60%and 95% and untreated wastewater. The untreated wastewater from thecentrifuge step is passed to a water treatment plant along pipe 66 withthe remaining sludge cake passed to a paddle mixer 67 along pipeline 68.Additives may be added to the sludge cake in the paddle mixer 67 beforebeing sent to concrete plant mixer 70 where the sludge cake that hadadditives mixed thereto such as a concrete hardener or bonding agent aremixed with a dry mix of concrete and aggregate. The alkali solutionitself has a pH of equal to or above 12.5. The dry mix of concrete andaggregate has already been thoroughly mixed at that stage prior to theaddition of the sludge cake. Finally, the concrete mix is passed to amould shed (not shown) along pipeline 71.

Referring to FIG. 2, there is shown a diagrammatic illustration of oneform of suitable apparatus for carrying out the method. Shreddedhousehold waste is fed from a container 1 to a mixing truck 2 by way ofconveyor 3. A conveyor 3 has load cells (not shown) connected thereto tocarefully monitor the amount of household waste being delivered to themixing truck 2. The household waste is placed in a paddle mixer 4 of themixing truck 2 wherein it is mixed with an alkali solution to form analkali and household waste mixture having a pH above 11.5. Once thehousehold waste and alkali solution have been mixed sufficiently, a pump5 on mixing truck 2 is actuated to pump the household waste and alkalimixture through a flexible hosepipe 6 to a mixing drum 7 of a nearbyconcrete mixing truck 8. The mixing drum 7 has already containedtherein, a thoroughly blended mixture of cement and aggregate. Once thealkali and household waste mixture has been added to the mixing drum 7containing the aggregate and cement, the mixing drum 7 is rotated,thereby blending the materials contained therein to form a concretemixture. Additional water may be added to the aggregate, cement andalkali and household waste mixture, if necessary.

One example of a concrete mixture made in accordance with the presentinvention will now be given:

EXAMPLE 1

One ton of household waste is shredded in a shredder so that the averageparticle size of the household waste is between 1 and 10 mm. Theshredded household waste is then mixed with 5 tons of alkaline solutionto form an alkaline and household waste mixture having a pH equal to orgreater than 11.5. At the same time, in a separate container, 6 tons ofcement are mixed with 36 tons of aggregate. In this case the aggregateis limestone. The cement and the aggregate are thoroughly mixed togetherto form a dry mix before the 6 tons of alkali and household wastemixture are added to the cement and aggregate to form the concretemixture. The concrete mixture is then left to set over a period of time.

Referring now to FIG. 3 of the drawings, there is shown a block diagramof the process of recycling household waste according to the inventionin step 20, refuse is received at the refuse disposal depot. In step 22,this refuse is separated by hand or other means into various differenttypes of refuse, including glass, plastics, paper and otherbiodegradable matter. In step 24, the biodegradable matter is placed ina shredder where the matter is broken up into smaller pieces. In step26, a further separation step may be carried out to remove any remainingsmall sized non-biodegradable materials from the household waste. Instep 28, an alkali solution is added to the shredded biodegradable wastematerial to further sanitise the biodegradable waste and form an alkaliand household waste mixture.

In step 30, in a separate mixing tank, cement and aggregate are mixedtogether thoroughly to form a dry mix. The alkali and household wastemixture is introduced into the separate mixing tank with the dry mix, instep 32, and they are blended together to form a concrete mixture instep 34. The concrete is then ready to be poured and made into concreteproducts or other similar articles.

The relative amounts of household waste, cement and aggregate aredetermined, depending on the strength and curing time requirements ofthe individual producing the concrete. The alkali solution blended withthe household waste can be a concrete hardener such as that sold underthe brand name Sika [Registered Trade Mark (RTM)]. The alkali willfurther act as a hardener assisting in the curing time of the concretemix once the alkali and household waste mixture and the dry mix havebeen blended together.

In addition to the alkali solution, a bonding agent such as those soldunder the Registered Trade Marks EVOSTICK, RONAFIX or POLYVINYL ACETATEis further added to the household waste sludge mixture to improve the pHvalue of the concrete to be produced, whilst also improving the bondingproperties of each of the main components in the concrete. The bondingagent typically will have a pH in the region of 8 to 11. This concretemix may then be used to construct road side barriers, concrete verges,and the like. Indeed, the concrete produced may be crushed and used asan aggregate for further concrete, made in accordance with this method.

A liquid detergent such as those produced by JEYES (RTM), DETTOL (RTM)or FLASH (RTM) may be added to the unsolidifled concrete mixture tofurther eliminate any residual bacteria present in the wastewatersludge. The concrete may then be poured into a heavy duty plasticcontainer and sealed therein to avoid any risk of contamination to theenvironment by leaching of the concrete once it has been exposed to theelements.

It will be understood that the entire process could be carried out inone large shed or three smaller sheds comprising an intake area, asorting and shredding area and a concreting plant and manufacturing arealinked by pipes or conveyors. The shed(s) could be provided withnegative pressure and/or extractor units if necessary. It is envisagedthat the mixing of the cement and aggregate could also be performed in astandard concrete mixing truck or in such similar device. The alkali andhousehold waste mixture could be added to the dry mix in the concretemixing truck, once any additional hardening agents or bonding agentshave been thoroughly mixed in with the alkali and household wastemixture. Reinforcing materials such as glass, fibre or steel can also beadded as part of the aggregate, further strengthening the concreteproduced. As an alternative to a heavy duty plastic container, afibreglass coating or plastic coating may be applied to concreteproduced in accordance with the method to add further protection andadditional strength to the concrete.

It is envisaged that the sludge cake or alkali and household wastemixture is added in sufficient quantities so that the sludge cake oralkali and household waste mixture forms between 5% and 50% of theconcrete mixture. Alternatively, the sludge cake or alkali and householdwaste mixture forms between 10% and 40% of the concrete mixture orindeed between 20% and 30% of the concrete mixture.

Once the concrete mixture has been allowed to set and harden over aperiod of time, the concrete produced by this method could be crushedand reused as an aggregate material thereby further using more of thebiodegradable household waste in the final concrete mixture produced andsafely disposing of higher levels of biodegradable household waste.

In this specification, the term “hardening agent” has been used todefine a substance that will reduce the time necessary for the concretemixture to set. The concrete bonding agent is used to describe asubstance that is used to enhance the cohesion of the individualingredients, once mixed. Potassium carbonate or aluminium silicate couldact as hardening components. The alkali solution used could be an alkalisolution of potassium hydroxide or alternatively sodium hydroxide,calcium hydroxide or barium hydroxide or other similar substance. In thedescription the alkali solution is said to have a pH of equal to orabove 12.5. It is envisaged that the alkali solution could have a pH ofequal to or above 13 or even 13.5. What is important is that the pHlevel is sufficient to provide an adequate kill of bacteria in a quickand efficient manner.

In the specification the terms “comprise, comprises, comprised andcomprising” or any variation thereof and the terms “include, includes,included and including” or any variation thereof are considered to betotally interchangeable and they should all be afforded the widestpossible interpretation and vice versa.

The invention is in no way limited to the embodiments hereinbeforedescribed, but may be varied in both construction and detail.

1-39. (canceled)
 40. A method of treating household waste comprising thesteps of: (a) separating any non-biodegradable waste above apredetermined size from the household waste for subsequent disposal; (b)shredding the remaining household waste in a shredder; (c) mixing theshredded household waste with an alkali solution to form an alkali andhousehold waste mixture having a pH above 11.5; (d) separating anyremaining metallic waste from the alkali and household waste mixture forsubsequent disposal; (e) separating any wood or plastics material fromthe alkali and household waste mixture for subsequent disposal; (f)removing any gases emitted by the alkali and household waste mixture;(g) subjecting the alkali and household waste mixture to a centrifuge toform sludge cake having a liquid content of between 60% and 95%, anduntreated water; and (h) mixing the sludge cake with a dry mix ofaggregate and cement to form a concrete mixture.
 41. A method oftreating household waste as claimed in claim 40, in which the step ofseparating any remaining metallic objects from the alkali and householdwaste mixture for subsequent disposal further comprises passing thealkali and household waste mixture through an eddy magnet.
 42. A methodof treating household waste as claimed in claim 40, in which the step ofseparating any wood or plastics material from the alkali and householdwaste mixture for subsequent disposal comprises passing the alkali andshredded household waste mixture through a floatation tank and skimmingthe wood and plastics material from the floatation tank.
 43. A method oftreating household waste as claimed in claim 40, in which the furtherstep is carried out of passing the untreated water to a water treatmentplant for sanitation and recycling.
 44. A method of treating householdwaste as claimed in claim 40, in which the additional step is carriedout of pouring the concrete mixture into plastic containers to preventinadvertent leaching of contaminants from the concrete mixture.
 45. Amethod of treating household waste as claimed in claim 40, in which thegases removed from the alkali and household waste mixture are passed toa burner for burning.
 46. A method of treating household waste asclaimed in claim 40, in which the step of mixing the shredded householdwaste with an alkali solution further comprises mixing the householdwaste with an alkali solution having a pH equal to or above 12.5.
 47. Amethod of treating household waste as claimed in claim 46, in which thealkali solution has a pH above
 13. 48. A method of treating householdwaste as claimed in claim 46, in which the alkali solution has a pHabove 13.5.
 49. A method of treating household waste as claimed in claim40, in which the sludge cake is added in sufficient quantities so thatthe sludge cake forms between 5% and 50% by weight of the concretemixture.
 50. A method of treating household waste as claimed in claim49, in which the sludge cake is added in sufficient quantities so thatthe sludge cake forms between 10% and 40% by weight of the concretemixture.
 51. A method of treating household waste as claimed in claim49, in which the sludge cake is added in sufficient quantities so thatthe sludge cake forms between 15% and 30% by weight of the concretemixture.
 52. A method of treating household waste as claimed in claim40, in which the aggregate and cement are mixed together in a separatecontainer prior to mixing with the sludge cake.
 53. A method of treatinghousehold waste as claimed in claim 40, in which the step of mixing thehousehold waste with an alkali solution further comprises mixing thehousehold waste with a concrete hardener.
 54. A method of treatinghousehold waste as claimed in claim 40, in which the additional step iscarried out of adding a bonding agent to the concrete mix.
 55. A methodof treating household waste as claimed in claim 54, in which the bondingagent has a pH in the region of 8 to
 11. 56. A method of treatinghousehold waste as claimed in claim 40, in which the aggregate comprisesone or more of grey wacke stone, sand, sandstone, gravel, limestone,crushed shale, crushed seashells, pencil, kiln dried sand, grit,pulverised fuel ash, slag from steelworks, and recycled crushedconcrete.
 57. A method of treating household waste as claimed in claim40, in which the method further comprises the additional step ofallowing the concrete mixture to set and after a predetermined length oftime, crushing the concrete mixture for subsequent re-use as anaggregate in the method.
 58. A method of treating household waste asclaimed in claim 40, in which the method further comprises the step ofadding water to the concrete mixture, on addition of the sludge cake tothe cement and aggregate mixture.
 59. A method of treating householdwaste as claimed in claim 40, in which the method further comprises thestep of adding a detergent to the concrete mixture prior to curing. 60.A method of treating household waste as claimed in claim 40, in whichthe sludge cake, cement and aggregate are mixed in a ratio of 1:1:6 byweight to form the concrete mixture.
 61. A method of treating householdwaste as claimed in claim 40, in which the method further comprises thestep of milling the household waste prior to separating any wood orplastics material therefrom.
 62. A method of treating household waste asclaimed in claim 40, in which the step of mixing the sludge cake withthe cement and aggregate to form a concrete mixture is performed in themixing drum of a concrete mixing truck.
 63. A method of treatinghousehold waste as claimed in claim 40, in which the sludge cake has asolids content of between 10 and 40%.
 64. A method of treating householdwaste as claimed in claim 63, in which the sludge cake has a solidscontent of between 10 and 25%.
 65. A method of using household waste inthe production of concrete comprising the steps of mixing cement,aggregate and household waste together to form a concrete mix,characterized in that the method further comprises the initial steps of:(a) shredding the household waste to a predetermined particle size; (b)mixing an alkali solution with the shredded household waste to form analkali and household waste mixture having a pH equal to or greater than11.5, prior to mixing with the cement and the aggregate.
 66. A method ofusing household waste in the production of concrete as claimed in claim65 in which the alkali solution is added to the shredded household wasteso that the alkali and household waste mixture has a liquid content byweight of between 60% and 90%.
 67. A method of using household waste inthe production of concrete as claimed in claim 65 in which the alkalisolution is added to the shredded household waste so that the alkali andhousehold waste mixture has a liquid content by weight of between 75%and 90%.
 68. A method of using household waste in the production ofconcrete as claimed in claim 65 in which the method further comprisesthe steps of: (c) passing the alkali and household waste mixture througha centrifuge to separate the alkali and household waste mixture intosludge cake having a liquid content by weight of between 60% and 90%,and untreated wastewater; (d) drawing off the untreated wastewater andleaving only the sludge cake; and (e) mixing the sludge cake with thecement and aggregate mixture.
 69. A method of using household waste inthe production of concrete as claimed in claim 68 in which the alkaliand household waste mixture are passed through a centrifuge until thesludge cake has a liquid content of between 75% and 85%.
 70. A method ofusing household waste in the production of concrete as claimed in claim65 in which the household waste is shredded to provide a shreddedhousehold waste having an average particle size of between 1 mm and 10mm.
 71. A method of using household waste in the production of concreteas claimed in claim 70 in which the household waste is shredded toprovide a shredded household waste having an average particle size ofbetween 3 mm and 8 mm.
 72. A method of using household waste in theproduction of concrete as claimed in claim 65 in which the alkalisolution added to the household waste is a concrete hardener.
 73. Amethod of using household waste in the production of concrete as claimedin claim 65 in which the method further comprises the step of adding abonding agent to the concrete mix.
 74. A method of using household wastein the production of concrete as claimed in claim 65 in which the stepof adding an alkali solution to the shredded household waste furthercomprises the step of adding an alkali solution having a pH equal to orabove 12.5.
 75. A method of using household waste in the production ofconcrete as claimed in claim 65 in which the alkali and household wastemixture comprises between 5 and 50% by weight of the concrete mixture.76. A method of using household waste in the production of concrete asclaimed in claim 75 in which the alkali and household waste mixturecomprises between 10% and 40% by weight of the concrete mixture.
 77. Amethod of using household waste in the production of concrete as claimedin claim 75 in which the alkali and household waste mixture comprisesbetween 15% and 30% by weight of the concrete mixture.
 78. A method ofusing household waste in the production of concrete as claimed in claim65 in which the alkali and household waste mixture, cement and aggregateare mixed in the ratio 1:1:6 by weight to form the concrete mixture.